1. Field of the Invention
This invention relates to a pulse-width modulation (PWM) control system for an electronic device, more particularly to such a control system which achieves high-response pulse-width modulation control of an electronic device such as a linear solenoid based on an accurate determination of the current flow.
2. Description of the Prior Art
As taught by Japanese Laid-Open Patent Application Nos. Hei 3(1991)-199757, Hei 3(1991)-153980, Hei 4(1992)-50550 and Hei 2(1990)-300556, for example, it is a common practice to detect the current flow through an electronic device such as a linear solenoid used in the hydraulic circuit of vehicle transmission and perform control by a pulse-width modulation or pulse-duration modulation based on the detected current flow.
When such an electronic device, particularly a vehicle linear solenoid is controlled through PWM, feed-back correction of the current flow becomes necessary owing to fluctuation in the voltage of the on-board power supply and fluctuation in the resistance of the electronic device with temperature.
Feedback control is generally conducted by detecting the current flow through the electronic device, comparing the detected value with the desired value, and using PID control law to determine a manipulated variable for eliminating the control error. As will be explained later with reference to FIG. 4, however, the instantaneous current in an electronic device varies constantly with the duty ratio. The detected value therefore differs depending on the sampling time point.
Because of this, the detection circuit is ordinarily provided with an RC circuit for smoothing the detected value, which is therefore obtained with a first-order lag. This does not cause a problem when the desired current is constant. In the case of stepped control in which the desired value is varied in steps, however, the delay of the first-order lag value relative to the actual current flow leads to the manipulated variable being determined by use of a large feedback gain for raising the control response. As explained later with reference to FIG. 6, this tends to cause overshooting of the controlled variable. Conversely, an attempt to avoid overshooting runs into the problem that it degrades control response because of the need to lower the feedback gain.